# B How long is a beam of sunlight?

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1. Oct 15, 2017

### SocratesRedux

How long is a beam of light or sunlight?

2. Oct 15, 2017

### Staff: Mentor

Whatever the distance is from the Sun to the point at which the beam is absorbed. Roughly 150 million kilometers if this beam is absorbed here on Earth, but can be many light-years long if it continues out into interstellar and intergalactic space.

3. Oct 15, 2017

### Staff: Mentor

Unless you aim at some dust cloud, visible light will typically spread for billions of years or even longer. There is light coming from the Sun 5 billion light years away from us.

4. Oct 15, 2017

### Staff: Mentor

Huh?

5. Oct 15, 2017

### Staff: Mentor

I think @mfb means if we could observe light right at this very minute and at huge distances we could see the light from the early solar system. Light emitted billions of years ago.

But what about that same 5 billion year old "ray" at the other end from the ancient starting point - does it still have a tail here at the sun? Ray, in English, implies some kind of continuity, I believe, through to infinity. See link below

I do not think there really is an answer. And language gets in the way of understanding.

This is a common accepted definition of a ray (mathematically):

Here is link so you can play with it:
http://study.com/academy/lesson/what-is-a-ray-in-geometry-definition-examples.html

6. Oct 15, 2017

### Staff: Mentor

The Sun is about 5 billion years old. 5 billion years it emitted light, and most of that light still exists - now 5 billion light years away from us. And in the meantime it continued to emit light of course, so we have a huge volume filled with light from the Sun.

That applies to all stars, of course, our Sun is not special in that way.

7. Oct 15, 2017

### Staff: Mentor

Okay, I misunderstood your post. I thought you were saying that there was sunlight coming to us (Earth) from 5 billion lightyears away. Hence my confusion.

8. Oct 15, 2017

### Bigjoemonger

The length of the beam is dependant on the distance it gets from the source before being absorbed by an object and how long the source emits the light.

9. Oct 16, 2017

### SocratesRedux

Barry -So a beam of light is infinite?

10. Oct 17, 2017

### Staff: Mentor

No, it has a finite length based on how long the light has been traveling. If I point a laser pointer upwards into space and turn it on for exactly one second, the beam has a length of one light-second and the head of the beam is $ct$ meters away, where $c$ is the speed of light (measured in meters) and $t$ is the time (measured in seconds) since I turned the laser on. The tail of the beam is then $c(t-1)$ meters away.

11. Oct 18, 2017

### SocratesRedux

One could then calculate that a beam of light from the big bang aftermath was something less than 13.6 Billion years old, but was still traveling further every second. BB+13.6Y+=Universe Edge. Is the light beam we see on earth 9 Billion years old?
Can we speculate that there is light emitting residue of the Big Bang still present at the "center" of the Universe?

12. Oct 18, 2017

### Staff: Mentor

There is no "center of the universe" and also no "edge". This thread might help.

We see light that is about 13.7 billion years old - the cosmic microwave background (not visible light any more due to the expansion of the universe), emitted about 380,000 years after the Big Bang.

13. Oct 18, 2017

### SocratesRedux

Sorry, your "logic" doesn't work for me. If we -our solar system-are only 4.5 Billion years old, then the light emitting from what ever time period you speculate must have "passed" us some 4.5 Billion years ago. As such, it would be the "edge" of our universe. The Planck institute has visualized an image of the shape of the universe, based on this concept, which stretches some 27+ Billion years side to side, slightly and horizontally oval., By my definition the edge of the universe is the front of the beams of light originating from the aftermath of the big bang -which created the physical laws of this universe-and therefore did indeed emit some forms of radiation, accepted by me to include visible light.

14. Oct 18, 2017

### Staff: Mentor

The age of our solar system is irrelevant in cosmology.

There is an edge of our observable universe, but that is only special for us, it is like the horizon if you are on a ship on an ocean. There is nothing special happening at the horizon, it is just how far you personally can see. Someone at the other side of our galaxy has a slightly different region of space as their observable universe.
What this image shows is the border of our observable universe, projected on a computer screen. It shows the full sphere of the sky around us, flattened like a map.
It is not useful if you start inventing new definitions.
There is no such thing. The light was emitted everywhere in the universe, and it is everywhere in the universe.
The Big Bang was not an explosion in space, it was the creation of space itself - it "happened everywhere at the same time" (don't take that too literally). There is no center, there is no edge, and there is not even a preferred position. The universe, on a large scale, looks the same for everyone everywhere.

15. Oct 18, 2017

### Staff: Mentor

Not true. The very early universe was filled with a hot, dense plasma of nuclei (protons and neutrons) and electrons. As the universe expanded, this plasma cooled off until about 380,000 years after the big bang when the temperature dropped low enough for electrons to combine with nuclei and form the first atoms. Prior to this point in time, this plasma formed a sort of "fog" that did not allow light to propagate more than a very short distance before being absorbed. After this recombination event, the universe suddenly became transparent to light, and the light that had been emitted just prior to this, along with the light then generated from the hot gas of atoms, was then able to pass freely through the universe.

Note that light was emitted everywhere in the universe at about the same time. The CMB we see now has been traveling for nearly 13.7 billion years from its origin to reach us. The fact that our solar system formed about 4 billion years ago (from pre-existing gas and dust) doesn't have anything to do with the CMB.

No, it's just from the "surface of last scattering". An imaginary surface from which the CMB that we see in the present was emitted. This surface moves outwards all the time, since we see light generated from points progressively further away in this "fog" as time passes. An analogy would be to imagine that you're standing on the top of a hill in the middle of an enormous city at night. At precisely the same instant, everyone in the city turns on their lights. From your point of view, since light moves at a finite speed, you would first see the houses near you light up. Then the houses just a bit further away. Then a bit further than that. The boundary between lit and unlit houses could be considered an imaginary boundary, which moves outward away from you as time passes.

Your definition is irrelevant. We stick to mainstream physics here at PF. Please read our terms and rules for more information.

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